As a disposable biosensor, a capillary type biosensor that is constituted such that a sample liquid is fed to a reaction part using a capillary phenomenon is known. FIG. 10 is an exploded perspective view of such a conventional biosensor 9, and FIG. 11 is a sectional view of the assembled biosensor 9. The biosensor 9 has a structure in which an insulating base 90, a spacer 91 and a cover 92 are built up on top of one another. As shown in FIG. 11, a capillary 93 having an inlet port 93a is defined by the spacer 91 between the base 90 and the cover 92. A working electrode 94, a counter electrode 95, and a reaction part 96, which is formed on top of the working electrode 94 and the counter electrode 95, are provided inside the capillary 93. The reaction part 96 contains any of various reagents necessary for a prescribed reaction system, for example a redox enzyme, in accordance with the subject of quantification.
With a biosensor 9 having such a structure, the sample liquid moves from the inlet port 93a through the capillary 93 due to a capillary phenomenon, and reaches the reaction part 96. The reagent contained in the reaction part 96 then dissolves in the sample liquid, and a redox reaction occurs. By measuring the oxidation current at this time, the concentration of a specific component in the sample liquid is quantified.
However, with the conventional biosensor 9, there has been a problem that if a solid component in the sample liquid is present in the vicinity of the working electrode 94 or the counter electrode 95, then the measured current value is affected, bringing about a worsening of the measurement precision.
Art of which an object is to avoid the effects on the measured current due to such a solid component is disclosed in Japanese Patent Application Laid-open No. 11-344461 Official Gazette. As shown in FIG. 12 of the present application, the biosensor disclosed in this official gazette has a structure in which a filter 97 constituted from a fibrous material such as fiberglass is further formed on top of the reaction part 96 of the biosensor of FIG. 11. The filter 97 is formed for example by making the fibrous material into a single body such as a fleece or a felt. The sample liquid that has moved through the capillary 93 thus dissolves the reaction part 96 after the solid component has been filtered out by the filter 97, and then reaches the working electrode 94 and the counter electrode 95. One would presume that it would be difficult to make the sample liquid flow adequately and filter out the solid component at the fibrous filter 97 in the case that the viscosity of the sample liquid is high, and hence according to Japanese Patent Application Laid-open No. 11-344461 Official Gazette, a surfactant is contained in the filter 97 to make the movement of the sample liquid through the filter 97 easy and make the filtering out of the solid component by the filter 97 progress smoothly.
However, if a surfactant is present in the filter 97, then in the case for example that whole blood is used as the sample liquid, hemolysis occurs when the blood passes through the filter 97, and hence the blood cell component that leaches out from the blood cells influences the measurement results. As a result, the accuracy of the measurements drops.
Moreover, the cover 92, which is one of the members defining the capillary 93, is generally formed from a resin material such as polyethylene terephthalate (PET). However, PET is a hydrophobic material, and hence there is a tendency for the movement of the sample liquid through the capillary 93 to be inhibited. When the sample liquid is taken into the capillary 93, a sufficient amount and a sufficient speed thus cannot be achieved, and hence a situation may arise in which the concentration of the specific component cannot be measured accurately.